1. Field of the Invention
This invention relates generally to an apparatus and method for reducing vibration in a component of a nuclear reactor.
2. Description of the Related Art
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and may be closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically may be spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and may be supported by a shroud support structure.
FIG. 1 is a schematic, partial cross sectional view, with parts cut away, of a reactor pressure vessel (RPV) 20 for a boiling water reactor. RPV 20 has a generally cylindrical-shape and may be closed at one end by a bottom head and at its other end by removable top head (not shown). RPV 20 may be filled with a reactor coolant fluid (not shown). A top guide (not shown) may be situated above a core plate 22 within RPV 20. A shroud 24 surrounds core plate 22 and may be supported by a shroud support structure 26. A downcomer annulus 28 may be formed between shroud 24 and sidewall 30 of RPV 20.
An annulet nozzle 32 extends through sidewall 30 of RPV 20 and may be coupled to a jet pump assembly 34. Jet pump assembly 34 may include a thermal sleeve 36 which extends through nozzle 32, a lower elbow (only partially visible in FIG. 1), and a riser pipe 38. Thermal sleeve 36 may be secured at a first end (not shown) to a second end of the lower elbow. The first end of thermal sleeve 36 may be welded to the second end of the lower elbow. A first end of the lower elbow similarly secured, or welded, to one end of riser pipe 38. Riser pipe 38 extends between and substantially parallel to shroud 24 and sidewall 30. A riser brace assembly 40 stabilizes riser pipe 38 within RPV 20. The riser brace assembly 40 may be fabricated of type 304 stainless steel which, after periods of use, may be susceptible to cracking at welded joints. The riser brace assembly 40 may be connected between riser pipe 38 and sidewall 30.
BWRs have emerged as a reliable type of nuclear reactor for producing electrical energy. However, some BWRs have experienced cracking in various components of the BWR. One contributing factor to component cracking in a BWR may be due to high cycle fatigue. Typically, a BWR may operate from about one to two years on a single core loading of fuel. Upon completion of a given period (known as an energy cycle or fuel cycle), approximately ¼ to ½ of the least reactive fuel (oldest or most burnt) may be discharged from the reactor. The number of cycles which may constitute a substantially high number of cycles may vary from BWR to BWR, as other factors may affect cycle time, such as design, operating conditions, etc.
High cycle fatigue may be caused, for example, by a substantially high acoustic frequency, for example a frequency above 100 Hz, and/or a substantially low frequency vibration, for example a frequency below 100 Hz. It should be understood that the frequency which constitutes a high and/or a low acoustic frequency may vary based on the application. The amplitude of a vibration in a BWR may directly influence or exacerbate high cycle fatigue, which in turn may cause the cracking of a component of the BWR. The amplitude of the vibration in the BWR experienced by a component of the BWR may be directly proportional to the stress in the component. High amplitude of the vibration in the BWR may lead to a high stress level, which may cause the cracking of a component of the BWR.
FIG. 2 is a detailed view of a conventional riser brace assembly. Riser brace assembly 40 primarily provides lateral support to the jet pump assembly 34 via riser pipe 38, and includes a riser brace block 43 and two riser brace leaves, an upper riser brace leaf 41 and a lower riser brace leaf 42. Leaves 41 and 42 are attached to riser brace block 43 by welds, and riser brace block 43 may be welded to a support pad 130 which in turn may be affixed to RPV sidewall 30. At the other end, leaves 41 and 42 of the riser brace assembly 40 are connected to a yoke such as brace plate 49. Brace plate 49 in turn may be welded to the riser pipe 38.
The riser brace assembly 40 may be designed to accommodate differential thermal expansion resulting from reactor start-up and heat-up, and to accommodate the flow-induced vibration that may be incumbent in the reactor water recirculation system (not shown) due to the reactor recirculation pumps. Accordingly, a concern with the riser brace assembly 40 is that the natural frequency of the riser brace assembly 40 be greater than the vane passing frequency of the recirculation pumps at any pump speed. If the vane passing frequency of the recirculation pumps equals or exceeds the natural frequency of the riser brace assembly 40, the riser brace assembly 40 may experience resonance, as the amplitude of the vibration increases. A substantially high amplitude of the vibration frequency in the riser brace assembly 40 may lead to high stress levels at the leaves 41 and 42 and/or at welds between leaves 41/42 and block 43 and/or sidewall 30, which may cause cracking at one or more locations in the riser brace assembly 40. A vibrating and/or crack-damaged riser brace assembly 40 could potentially cause the riser pipe 38 to become unstable, adversely affecting the jet pump assembly 34.